Injection and transfer molds are commonly used for encapsulating electronic devices in plastic, especially semiconductor devices. The active element of the device is typically mounted on a leadframe which provides the external connections for the finished device. Usually, a leadframe containing many active elements is placed in a recess in one part of a separable mold. When the mold is closed, a cavity is formed surrounding each active element. The separable mold parts press tightly against the leadframe around the perimeter of each cavity in order to seal each cavity. Runners, channels, and gates connect one or more sources of plastic molding compound to each of these cavities. Plastic in liquefied form is forced into each cavity and around the active element where it hardens to form the plastic encapsulation. Thermoplastic and thermosetting plastic materials are commonly used. Type MG15F-0140 molding compound made by HYSOL Division of Dexter Industries, Olean, N.Y. is an example of a suitable thermosetting material. Suitable thermoplastic materials are well known.
A problem with such molds and molding methods is the occurrence of flash. Flash is unwanted plastic, typically in the shape of thin sheets or webs, which forms between the mold halves or mold parts, and/or on the leads, in locations where no plastic is desired. Flash is undesirable since additional effort must be expended to remove it from the leadframes after molding and to clean away fragments which may have stuck to the mold or dropped onto the mold during unloading of the leadframes. If the flash is not removed from the mold prior to the next shot, then hardened flash may cause coining of the mold and shorten its working life. This problem is most troublesome in the region of the mold chases, i.e., the portions of the mold surrounding the leadframe regions and including the sealing surfaces and various edges
Thus, there is a continuing need for improved mold designs and molding methods which reduce the amount of flash which occurs during molding and which more fully insure that flash which is formed is removed along with the solidified excess plastic left (after molding) in the channels, runners, and gates which connect the plastic sources to the cavities.
Accordingly, it is an object of the present invention to provide an improved molding means and method wherein the mold is largely self-cleaning and the amount of flash remaining after removal of the solidified excess plastic is much reduced.
It is a further object of the present invention to provide an improved molding means and method wherein the substantially self-cleaning feature is suitable for use with existing molds by simple modification of their chase and channel shapes.
It is an additional object of the present invention to provide an improved molding means and method wherein the substantially self-cleaning feature is provided without requiring additional mold area.
It is a further object of the present invention to provide an improved molding means and method wherein the self-cleaning feature is suitable for use substantially everywhere it is desired to prevent plastic from encroaching between closed mold parts.
It is an additional object of the present invention to provide an improved molding means and method wherein the lateral edges of the channels, runners, and gates are self-sealing, thereby reducing the formation of flash at the edges of such features.
It is a further object of the present invention to provide an improved molding means and method which prevents plastic from extruding into narrow spaces left between the closed mold faces due to leadframe tolerance requirements.